1. Field of the Invention
The present invention relates to a magnetic head of the type which comprises a magnetic core, a casing for enclosing the core and a fixing material used to secure the core within the casing.
More particularly, the present invention relates to an improvement in such type of magnetic head by adding filler(s) to the fixing material so as to uniformize the abrasion of the tape sliding surface and thereby to lengthen the lifetime of the head.
2. Description of the Prior Art
During the use of a magnetic head over an extended period of time, the tape sliding surface thereof is gradually abraded by the friction between the surface and a magnetic tape sliding on it. At an early stage in the abrasion, the magnetic head begins to deteriorate in its recording characteristic at the high frequency band. At a further advanced stage in the abrasion, the deterioration in the recording characteristic spreads over the whole range of full frequency band.
Conventionally, permalloy is mainly used as the core material for a magnetic head. To solve the problem of the above-mentioned abrasion, there also have been developed magnetic heads in which a core made of material of higher abrasion resistance such as hard permalloy, ferrite, sendust or the like is used. However, the surface of a magnetic head on which a magnetic tape slides is formed not only by the core material but also by the fixing material injected into the magnetic head to secure the core within the casing. Therefore, to maintain the original characteristic or performance of a magnetic head, it is absolutely necessary to take into consideration the balance in abrasion between the core and the fixing material. Thus, a measure is required to equalize the abrasion on the core and that on the fixing material so that the tape sliding surface may always be kept flat. For this purpose, the fixing material used in a magnetic head is generally prepared by adding some amount of filler to resin.
A most common filler used for this purpose is SiO.sub.2. By adding about 10-30% by weight of fine powder of SiO.sub.2 based on the weight of the whole of the fixing material, the equalization of abrasion on both the core and the fixing material may be attained. But, such balancing of abrasion between the core material and the fixing material is not the sole factor depending upon which the kind and amount of filler to be used is determined. There are various other factors such as the displacement of the core within the change of temperature and humidity (caused by the expansion or shrinkage of the core and the fixing material), the injectability of fixing material, the adhesive property of the fixing material and the like. The use of SiO.sub.2 fine powder less than 30% by weight based on the weight of the whole fixing material makes the magnetic head too sensitive to temperature and humidity. On the other hand, the use of more than 80% by weight based on the weight of the whole fixing material makes the injectability and adhesive property of the fixing material inadequately low. For these reasons, the amount of SiO.sub.2 powder hitherto used for conventional magnetic heads is in the range of from 30 to 80 percent by weight.
It is also generally known that in addition to SiO.sub.2 (quartz), there may be used other filler material such as Na.sub.2 O--CaO--SiO.sub.2 (soda glass), Mg.sub.3 [(OH).sub.2 Si.sub.4 O.sub.10 ] (talc), C(graphite), Al.sub.4 (OH).sub.8 Si.sub.4 O.sub.10 (kaolinite), Ca(SO.sub.4)2H.sub.2 O(gypsum), Al.sub.2 O.sub.3 (alumina) and the like. However, it has been found impossible to obtain a good fixing material by merely mixing materials selected from the above mentioned group of filler materials by changing their contents in the fixing material. Thus, no combination thereof could produce a fixing material which was excellent in injectability and adhesive property and by which the trouble of core displacement occurring with the change of temperature and humidity could be minimized; and also a sufficiently uniform abrasion of the tape sliding surface sufficient to keep it flat could not be attained. With respect to the abrasion, it has been recognized that the conventional magnetic head, the fixing material of which contains 30 to 80% by weight of fine powder of SiO.sub.2 can not be always maintained in the optimum condition.
During its use over a long period of time, the flat tape sliding surface of the magnetic head is changed into an uneven surface which in turn prevents a good contact between the magnetic tape and the magnetic core. Of course, this is caused by the fact that the abrasion of the core is larger than that of the fixing material. As a result, the electromagnetic transforming characteristic of the magnetic head is deteriorated due to a so-called spacing-loss in the range of high frequency at the beginning and further over the whole range of frequency later and thereby the output of the head is gradually reduced with the progress of the abrasion.
Moreover, in case of a cassette tape and similar tape recording apparatus, there is mounted in the cassette a tape pad by which the magnetic tape is brought into contact with the magnetic head under pressure. Therefore, the abrasion at the portion of magnetic core may reach a deep area of it and the irregularity of the tape sliding surface becomes extremely large with time. This will result in an undue undeterioration of the electro-magnetic transforming characteristic before the magnetic core reaches its limit of abrasion.
Further, even if any suitable filler could be found as an effective additive to the fixing material, such filler usually would have the drawback that an increase in the amount to be added adversely affects the adhesive property and the insulating property of the resulting fixing material.
Therefore, a development of a good fixing material capable of satisfying all the requirements described above has been desired earnestly.